We wish to develop a systematic procedure allowing the identification of lesions occuring in DNA to the extent of 100 parts per million or less. In the first stage of investigation, we are concentrating upon one particular type of damage: covalent cross-links between the two chains of DNA. A number of reagents have been described which cross-link DNA, but the nature of the cross-link is unknown in most cases. We are currently working with two cross-linking agents: nitrous acid and formaldehyde. Hydrolysis of the cross-linked DNA with venom exonucleases and deoxyribounuclease I, followed by DEAE-Sephadex chromatography, affords mononucleotides and a series of resistant oligonucleotides. The cross-links are isolated from the oligonucleotide fractions, using further hydrolysis, and liquid chromatography. The structures of cross-linked nucleosides isolated are being studied, using microchemical techniques and mass spectroscopy.